1. Field of the Invention
This invention relates to an image signal reproducing apparatus having a synchronizing signal generator for generating various kinds of timing signals in accordance with synchronizing signals recorded on a recording medium together with information signals.
2. Description of the Prior Art
There have been information signal recording and reproducing apparatuses in which synchronizing signals are recorded on a recording medium together with information signals when recording. The synchronizing signals recorded on the recording medium together with the information signals are reproduced when reproducing, various kinds of timing signals are generated in accordance with the reproduced synchronizing signals, and various kinds of processings are performed using the reproduced information signals in accordance with the generated timing signals.
In the above-described information signal recording and reproducing apparatuses, information signals to be recorded and reproduced are, for example, television signals.
As the current scanning method for television signals, a 2:1 interlaced scanning method is generally used in which scanning is performed by jumping every other scanning line. In the NTSC method, for example, interlaced scanning for television signals is performed so that scanning lines mutually interlace every 1/60 second, and a frame image for one picture frame is provided every 1/30 second using two field images each formed by a single interlaced scanning operation.
As described above, in the television signal in the NTSC method, one picture frame is provided by two field images. The single picture frame comprises 525 scanning lines.
A composite synchronizing signal (C.sub.sync) composed of a horizontal synchronizing signal (H.sub.sync) indicating one horizontal scanning period and a vertical synchronizing signal (V.sub.sync) indicating one vertical scanning period is added to the above-described television signal, and continuity of the synchronous signals is maintained at the boundary of consecutive field periods.
As an apparatus for recording the above-described television signals on a recording medium, such as a magnetic disk or the like, and reproducing the signals from the medium, there has been used a still video recording and reproducing apparatus,
In the still video recording and reproducing apparatus, a magnetic disk, serving as a recording medium, is rotated at a speed of 3600 rpm, an image signal for one field is recorded on one of a plurality of tracks concentrically formed on the magnetic disk, and the recorded image signal for one field is repeatedly reproduced to obtain a still picture image signal.
When reproducing the still picture image signal as described above, in order to conform to the above-described television-signal scanning method, an image signal for one field is delayed by 1/2 the horizontal synchronizing period (1/2 H) except for the vertical synchronizing period using a 1/2 H delay device, and an image signal delayed by the delay device and an undelayed image signal are output by being alternately switched at every one field period. Thus, a so-called skew compensation processing is performed which conforms to the above-described interlaced scanning while maintaining the continuity of the horizontal synchronizing signal.
FIG. 1 is a digram showing the schematic configuration of a conventional still video reproducing apparatus. The operation of the conventional still video reproducing apparatus will now be explained.
In FIG. 1, a magnetic disk 100 is rotated by a motor 101 at a speed of 3600 rpm. Signals recorded on tracks formed on the magnetic disk 100 by a magnetic head 102 are reproduced and supplied to a reproducing amplifier 103.
A signal reproduced by the magnetic head 102 is amplified to an amplitude level for practical use by the reproducing amplifier 103. The amplified signal is supplied to a demodulation circuit 104, whereby the signal is demodulated. An image signal for one field to which a composite synchronizing signal has been added is then output and supplied to a synchronizing signal separation circuit 105 and an image signal processing circuit 107.
The synchronizing signal separation circuit 105 separates the composite synchronizing signal from the signal supplied from the demodulation circuit 104. The separated composite synchronizing signal is supplied to a timing signal generation circuit 106 and a synchronizing signal addition circuit 108.
A magnetic piece (PG pin) 107 for detecting the rotation phase of the magnetic disk 100 is provided on a circumference on the core of the magnetic disk 100. Every time the PG pin 107 crosses over a PG coil 108 due to the rotation of the magnetic disk 100, a pulse signal is output from the PG coil 108. The pulse signal output from the PG coil 108 is subjected to waveform shaping by a waveform shaping circuit 109, and then is supplied to the timing signal generation circuit 106 as a PG pulse signal synchronizing with the rotation phase of the magnetic disk 100.
An image signal recorded on each track on the magnetic disk 100 is recorded so that the added vertical synchronizing signal is situated at a position deviated by 7H.+-.2H in the circumferential direction from the position where the PG pin 107 is provided.
The timing signal generation circuit 106 forms a blanking signal B and a skew compensation gate signal S in synchronization with the composite synchronizing signal supplied from the synchronizing signal separation circuit 105 and the PG pulse signal supplied from the waveform shaping circuit 109. The blanking signal B and the skew compensation gate signal S thus formed are supplied to an image signal processing circuit 110 and a change-over switch 113 (to be described later), respectively. The image signal processing circuit 110 performs horizontal and vertical blanking processings for the image signal supplied from the demodulation circuit 104 in accordance with the blanking signal B supplied from the timing signal generation circuit 106, and supplies the resultant signal to a synchronizing signal addition circuit 111.
The synchronizing signal addition circuit 111 adds the composite synchronizing signal separated from the synchronizing signal separation circuit 105 to the image signal for one field subjected to the blanking processings in the image signal processing circuit 110 in the preceding stage, and supplies the resultant signal to a 1/2 H delay circuit 112 and terminal "a" of the change-over switch 113. The 1/2 H delay circuit 112 delays the supplied image signal by a period of 1/2 H, and supplies the delayed image signal to terminal "b" of the change-over switch 113.
By alternately switching the connection between the side of terminal "a" and the side of terminal "b" at every one-field period in accordance with the skew compensation gate signal S output from the timing signal generation circuit 106, a frame image signal conforming to the interlaced scanning method is output from the change-over switch 113 via an output terminal 114.
FIGS. 2(a) and 2(b) show an example of the configuration of a synchronizing signal generator in the above-described still video recording and reproducing apparatus, and a timing chart indicating the operation thereof, respectively.
FIG. 2(a) is a diagram showing the configuration of a synchronizing signal generator for generating window pulses (H.sub.BLK) for providing horizontal blanking periods in the reproduced still-picture image signal in the still video recording and reproducing apparatus.
In the still video recording and reproducing apparatus, a signal C.sub.sync (see FIG. 2(b)) reproduced when reproducing is supplied to reset terminal RES of a counter 300 shown in FIG. 2(a).
The counter 300 is reset during a low-level period of the signal C.sub.sync supplied from the terminal RES, performs counting from a leading edge (T.sub.0 in FIG. 2(b)) of the signal C.sub.sync for a predetermined period (T.sub.2 in FIG. 2(b)), and supplies AND gates 301 and 302 with count data.
The AND gate 301 detects whether or not output count data from the counter 300 have reached T.sub.1 from T.sub.0 in FIG. 2(b), and outputs a high-level signal when data have reached T.sub.1. The AND gate 302 detects whether or not the output count data from the counter 300 have reached T.sub.2 from T.sub.1 in FIG. 2(b), and outputs a high-level signal when the data have reached T.sub.2.
Signals output from the AND gates 301 and 302 are supplied to set terminal S and reset terminal R of an S-R flip-flop 303, respectively. As a result, window pulses H.sub.BLK as shown in FIG. 2(b) are output from output terminal Q of the S-R flip-flop 303. In the still video recording and reproducing apparatus, horizontal blanking periods are provided in the still-picture image signal in accordance with the pulses H.sub.BLK formed as described above.
In the prior art as described above, however, a point to start recording for an image signal recorded on a magnetic disk and a horizontal synchronizing signal obtained from an image signal reproduced from the magnetic disk do not always maintain a constant phase relation. Furthermore, variations exist among reproduced magnetic disks in positional relation between the point to start recording of an image signal and the switching point between a signal delayed by 1/2 H from the image signal reproduced from the magnetic disk and an undelayed signal in skew compensation, that is, the position of a PG pin. Hence, when the signal delayed by 1/2 H from the image signal reproduced from the magnetic disk and the undelayed signal are switched at the position of the PG pin for the purpose of skew compensation, the phase between the point to start recording of the image signal recorded on the magnetic disk and the horizontal synchronizing signal obtained from the image signal reproduced from the magnetic disk is different for every magnetic disk.
FIG. 3, composed of FIGS. 3(a)-3(d), illustrates a timing charts indicating the waveforms of synchronizing signals added to an image signal reproduced from a magnetic disk for the purpose of explaining the above-described problem.
FIG. 3(a) is a diagram continuously showing a synchronizing signal added to an image signal reproduced from a magnetic disk, wherein point A indicates a point to start recording. FIG. 3(b) shows a signal delayed by 1/2 H from the signal shown in FIG. 3(a).
Skew compensation is performed by switching the signals shown in FIGS. 3(a) and 3(b) every time the magnetic disk performs a single rotation. FIG. 3(c) shows a waveform when the signal shown in FIG. 3(a) is switched to the signal shown in FIG. 3(b), and FIG. 3(d) shows a waveform when the signal shown in FIG. 3(b) is switched to the signal shown in FIG. 3(a).
As shown in FIGS. 3(c) and 3(d), extra pulses as shown by points B and C are produced when skew compensation is performed, and the continuity of the horizontal synchronizing signal is not maintained at these points in the prior art.
Furthermore, in the conventional apparatus, when the track to be reproduced is changed from an unrecorded track to a recorded track, or from a track on which an image signal corresponding to an odd-numbered field is recorded to a track on which an image signal corresponding to an even-numbered field is recorded (or vice versa), discontinuity of the horizontal synchronizing signal occurs in the reproduced still-picture image signal. For example, when the reproduced still-picture image signal is supplied to an external apparatus, such as a monitoring apparatus or the like, the external apparatus operates in accordance with the synchronizing signal added to the supplied still-picture image signal. Since the added horizontal synchronizing signal is discontinuous, there is the possibility that the picture frame made from the supplied still-picture image will be disturbed.
Moreover, since the composite synchronizing signal added to the reproduced image signal is reproduced from the magnetic disk, there is the possibility that a part of the composite synchronizing signal will be missing due to dropouts caused by the adherence of dust, scratches and the like, or that the signal will be deteriorated due to the penetration of noise from the outside, uneven rotation of the magnetic disk and the like.
In the prior art, when an image signal reproduced from a magnetic disk by the above-described still video reproducing apparatus is displayed as a still picture using, for example, a monitoring apparatus, it is possible to display a stable still picture even if the composite synchronizing signal added to the reproduced image signal is deteriorated as described above, because an AFC (automatic frequency control) circuit is provided in the monitoring apparatus. However, when a reproduced image signal to which a deteriorated composite synchronizing signal has been added is input to an image input apparatus (for example, an image memory apparatus or the like) not having the AFC circuit, various kinds of synchronizing signals formed according to the deteriorated composite synchronizing signal also become incorrect. As a result, the reproduced still-picture image signal processed according to the incorrect synchronizing signals is also deteriorated. Hence, the input of a normal image signal was not performed in some cases.